Define a variable to distinguish system_icu from bundled_icu in Blink

Source/config.gyp

 #
 # Copyright (C) 2013 Google Inc. All rights reserved.
 #
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 # notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
@@ skipping 99 matching lines @@
             'VCCLCompilerTool': {
               'AdditionalOptions': ['<!@(python ../../../tools/clang/scripts/blink_gc_plugin_flags.py enable-oilpan=<(enable_oilpan) <(blink_gc_plugin_flags))'],
             },
           },
         }],
         ['blink_disable_partition_allocator==1', {
           'defines': [
             'MEMORY_TOOL_REPLACES_ALLOCATOR',
           ],
         }],
+        ['use_system_icu==1', {
+          'defines': [
+            'USING_SYSTEM_ICU',
+          ],
+        }],
       ],
     },
   },
   {
     'target_name': 'unittest_config',
     'type': 'none',
     'dependencies': [
       'config',
       '<(DEPTH)/testing/gmock.gyp:gmock',
       '<(DEPTH)/testing/gtest.gyp:gtest',
     ],
     'export_dependent_settings': [
       'config',
       '<(DEPTH)/testing/gmock.gyp:gmock',
       '<(DEPTH)/testing/gtest.gyp:gtest',
     ],
     'direct_dependent_settings': {
       'variables': {
         'chromium_code': 1,
         'clang_warning_flags_unset': [ '-Wglobal-constructors' ],
       },
     },
   }
   ],
 }